SiC matrix fuel cladding tube with spark plasma sintered end plugs

ABSTRACT

A method of providing an end-capped tubular ceramic composite for containing nuclear fuel ( 34 ) in a nuclear reactor involves the steps of providing a tubular ceramic composite ( 40 ), providing at least one end plug ( 14, 46, 48 ), applying ( 42 ) the at least one end plug material to the ends of the tubular ceramic composite, applying electrodes to the end plug and tubular ceramic composite and applying current in a plasma sintering means ( 10, 50 ) to provide a hermetically sealed tube ( 52 ). The invention also provides a sealed tube made by this method.

BACKGROUND

1. Field

The invention relates to a fuel rod cladding tube made out of siliconcarbide (herein referred as SiC), regardless the cladding designarchitecture (monolithic, duplex with monolithic SiC on the inside and acomposite made with SiC fibers and SiC matrix on the outside, etc.) witha spark plasma sinter sealed end plug.

2. Description of Related Art

In a typical nuclear reactor, such as a pressurized water (PWR), heavywater (such as a CANDU) or a boiling water reactor (BWR), the reactorcore includes a large number of fuel assemblies, each of which iscomposed of a plurality of elongated fuel elements or fuel rods. Thefuel rods each contain nuclear fuel fissile material such as at leastone of uranium dioxide (UO₂), plutonium dioxide (PuO₂), uranium nitride(UN) and/or uranium silicide (U₃Si₂); with possible additions of, forexample, boron or boron compounds, gadolinium or gadolinium compoundsand the like either on or in pellets, usually in the form of a stack ofnuclear fuel pellets, although annular or particle forms of fuel arealso used. The fuel rods have a cladding that acts as a containment forthe fissile material. The fuel rods are grouped together in an arraywhich is organized to provide a neutron flux in the core sufficient tosupport a high rate of nuclear fission and thus the release of a largeamount of energy in the form of heat. A coolant, such as water, ispumped through the core in order to extract the heat generated in thecore for the production of useful work. Fuel assemblies vary in size anddesign depending on the desired size of the core and the size of thereactor.

The cladding on the fuel rods is usually made from zirconium (Zr) withup to about 2 wt. % of other metals such as Nb, Sn, Fe and Cr. Suchzirconium alloy clad tubes are taught, for example, by Biancheria etal., Kapil and Lahoda (U.S. Pat. Nos. 3,427,222; 5,075,075; and7,139,360, respectively). The fuel rods/cladding have an end cap at eachend and a hold down device such as a metal spring to keep the stack ofnuclear fuel pellets in place. FIG. 1 illustrates this type of prior artdesign, showing a string of fuel pellets 10, a zirconium-based cladding12, a spring holdown device 14, and end caps 16.

There are problems associated with metal clad fuel rods. They can wearif contacted by debris that may be present in the cooling watermentioned before. Under severe conditions such as “beyond design basis”accidents; metal cladding can react exothermally with steam at over1,093° C. (2,000° F.). These zirconium cladding metals protecting thenuclear fuel may lose strength during “a loss of coolant” accident,where reactor temperatures can reach as high as 1,204° C. (2,200° F.),and expand due to internal fission gases within the fuel rod. Inaddition, continuing utility industry demands have pushed reactoroperating temperatures and cladding radiation exposure to extremelimits.

All this has prompted considering use of experimental ceramic typematerials such as silicon carbide (SiC) monolith, fibers and theircombinations as taught by Maruyama et al. (U.S. Pat. No. 6,246,740),Zender, (U.S. Pat. No. 5,391,428), Hanzawa et al., (U.S. Pat. No.5,338,576); Feinroth (U.S. Pat. No. 5,182,077 and U.S. PatentPublication No. 2006/0039524 A1), Easier et al. (U.S. Patent PublicationNo. 2007/0189952 A1); and tangentially Korton, (U.S. Pat. No. 6,697,448)as complete or partial substitutes for metal fuel rods.

The absolutely right combination must be sought in the nuclear industryto make usually brittle ceramic, much more flexible, to relievestress/temperature/pressure in full failure conditions. One possibilityis use of experimental SiC fiber reinforced SiC composites; a two orthree-layer tube of high purity beta or alpha phase stoichiometricsilicon carbide covered by a central composite layer of continuous betaphase stoichiometric silicon carbide fibers infiltrated with beta phaseSiC and, in the case of three layers, an outer protective layer of finegrained beta phase silicon carbide. It has been suggested to pre-stressthe fiber component, forming the fibers into tows and tow reversewinding overlapping; where the fibers are coated with a less than amicrometer of SiC or carbon or graphite or boron nitride to provide aweak interface allowing slippage, all this to get better strainresistance and flexibility. Feinroth et al. in U.S. Patent PublicationNo. 2006/0039524 A1, herein incorporated by reference, describes suchnuclear fuel tubes and a matrix densification using well known processesof chemical vapor infiltration (CVI), polymer impregnation, andpyrolysis (PIP). Alumina (Al₂O₃) fibers in an alumina matrix have alsobeen suggested as a substitute.

As used herein, the term “Ceramic Composite” will mean and is defined asall of the above described composite type structures including SiC andAl₂O₃.

Surprisingly, very little is said about the end plugs for such ceramiccomposites. In fact, finding a sealing technology that attaches an endplug, and ensures hermeticity for a Ceramic Composite cladding, such asa silicon-carbide fuel rod cladding has been a very elusive task so far,due to the various requirements placed on such an interface joint thatis to:

-   -   ensure mechanical strength during and after normal operation,        anticipated operational occurrences, infrequent accidents, and        limiting faults;    -   ensure the hermeticity of the end plug-to-cladding joint under        irradiation and the nuclear reactor-specific corrosive        environment;    -   allow the joining process to accommodate fully-loaded cladding        (with fuel pellets and a hold down device). In addition, the end        plug and sealing technology must allow for pressurization of the        fuel rod with Helium or other thermally conductive backfill gas        at pressures typically up to 300 psi; and    -   allow the joining process to accommodate economy-of-scale        commercial use.

Several sealing technologies have been explored in the recent past; butto date none of them proved successful in a nuclear environment, whichis essential here. Thus, there are a number of sealing technologiessuggested, that use various compounds (other than SiC) to seal SiC parts(e.g., Ti-based formulations, Al—Si formulations), including brazing andother techniques, for example: V. Chaumat et al, U.S. Patent PublicationNo. 2013/0004325A1; A. Gasse, U.S. Patent Publication 2003/0038166; A.Gasse et al., U.S. Pat. No. 5,975,407; F. Montgomery et al., (U.S. Pat.No. 5,447,683); G. A. Rossi et al., (U.S. Pat. No. 4,925,608); andMcDermid, “Thermodynamic brazing alloy design for joining siliconcarbide,” J. Am. Ceram. Soc., Vol. 74, No. 8, pp. 1855-1860, 1991.

There has been an explosion of research on SiC joining technology since2007; for example: C. H. Henager, Jr. et al., “Coatings and joining forSiC and SiC composites for nuclear energy systems,” Journal of NuclearMaterials, 367, 370 (2007) 1139-1143; M. Ferraris et al., “Joining ofmachined SiC/SiC composites for thermonuclear fusion reactors,” Journalof Nuclear Materials, 375 (2008) 410-415; J. Li et al., “A hightemperature Ti—Si eutectic braze for joining SiC,” Materials Letters, 62(2008), 3135-3138; W. Tian, “Reaction joining of SiC ceramics usingTiB₂-based composites,” Journal of the European Ceramic Society, 30(2010) 3203-3208 and M. Ferraris et al., “Joining of SiC-based materialsfor nuclear energy applications,” Journal of Nuclear Materials, 417(2011) 379-382. These articles are attempting to apprise utilities, ofmeans to ensure higher and higher outputs. The utilities are requiringmore and more stressed designs and materials as is economicallynecessary to meet world energy needs.

The above ceramic models are no longer experiments and are generallyshown to have high mechanical strength, and are thought capable ofrealizing the required gas-tightness for a nuclear reactor; however,these joining technologies failed to show the corrosion and irradiationresistance necessary to survive in a nuclear reactor environment for atypical lifetime of a fuel rod. Other sealing technologies (e.g.,experimental spark plasma sintering—hereinafter “SPS”) described byMunir et al., “The effect of electric field and pressure on thesynthesis and consolidation of materials herein incorporated byreference, describes: a review of the spark plasma sintering method,” J.Mater Sci., 41 (2006) 763, 777. They make no use of additional chemicalcompounds; but economical large-scale manufacturing using this processare illusive to this date and still remains a challenge. Hot isostacicpressure (HIP), a well known technique for use in many commercial areas,can also be used to join SiC to SiC; but HIP, noted previously, A Rossiet al., U.S. Pat. No. 4,925,608, is not practical in the fragileenvironment of sealing nuclear fuel rods due to the long sintering cycleand high temperatures, about 1,700° C., and extremely high pressure, andis not applicable to mass production. What is needed is a commerciallyviable joining method for sealing tubular ceramic composites with endcaps of ceramic or metal.

It is a main object of this invention to provide a method for producinghigh strength, hermetically sealed, commercially useful and viable endplug seals and methods, resistant to irradiation in a dramatic nuclearenvironment using ceramic composite tubes as a basis for containing thefuel pellets.

SUMMARY

This invention, in order to solve the above problems and provide themain object, is related to a method of providing an end-capped tubularceramic composite which can be filled with nuclear fuel, comprising thesteps of:

-   -   (1) providing a tubular Ceramic Composite having tube walls and        a circumferential axis and separately at least one end plug        material;    -   (2) applying the at least one end plug material, preferably a        finely machined sanded material, to at least one end of the        tubular ceramic composition, the end plug having an exterior and        interior side contacting the tubular ceramic composite at a        plug/cladding interface;    -   (3) applying at least one primary electrode to the exterior side        of the at least one end plug;    -   (4) optionally applying at least one secondary electrode to the        exterior side of the tubular ceramic composite; and    -   (5) applying electrical current to the electrodes present; using        spark plasma sintering to supply a rapid temperature rise in the        plug/cladding interface of up to 1,500° C./min.; where the        interface temperatures at the end plug interface with the        cladding tube end are ambient to 2,500° C.

The invention in a more detailed, preferred form also relates to amethod comprising the steps of:

-   -   (1) providing a tubular Ceramic Composite having tube walls and        a circumferential axis and separately at least one end plug        material;    -   (2) applying the at least one end plug, preferably a finely        machined sanded ceramic or metal end plug composition or ceramic        precursor composition material, to at least one end of the        tubular ceramic composition, the end plug having an exterior and        interior side contacting the tubular ceramic composite at a        plug/cladding interface;    -   (3) applying at least one primary electrode to the exterior side        of the at least one ceramic end plug;    -   (4) optionally applying at least one secondary circumferential        electrode to the exterior side of the tubular ceramic composite;        and    -   (5) applying electrical current to the electrodes present using        a spark plasma sintering technique to supply a rapid temperature        rise at the plug/cladding interface of up to 1,500° C./min.;        where interface temperatures at the end plug-tube interface with        the cladding tube end are from ambient to 2,500° C., applied        from 0.01 to 6.0 minutes at pressures from 0.001 MPa to 50.0 MPa        to hermetically seal the tube to the at least one end cap. The        average current is 200 A-800 A (amps) and the peak pulsed        voltage is 2-4 V (volts) and the time is preferably 5 minutes to        60 minutes.

The ceramic end caps are preferably the same or a precursor of the samematerial as the ceramic composite which will transition to that materialupon heating.

The operating parameters of the generally known spark plasma process(SPS process), except for particular critical ranges disclosed anddiscovered herein, are preferably: applying temperature rises at theplug/cladding gaps/interface of up to 1,500° C./min., at a rapid rate ofpreferably 100° C./min. to 1,000° C./min., most preferable 1,000°C./min. and up to a peak temperature of 2,500° C., preferably 1,800° C.to 2,150° C., most preferably 2,100° C. at 1.0 to 6.0 minutes and 5.0 to10 MPa. As shown in FIG. 2 vs. FIGS. 3-4, the circumferential electrode24 is optional but preferred. The simpler technique of FIG. 2, uses justtwo end electrodes 22. Both electrical applications are useful. Also,preferably a bonding pressure is applied after step (4) of 4 MPa to 20MPa with a hold time of 5 minutes to 60 minutes; and an inner gas suchas helium is inserted/injected/pumped into the tube after step (1) orstep (2) possibly by drilling into the end plug, inserting the gas underpressure and sealing the end plug tube at from 50 to 500 psi.

The preferred end caps are preferably made of SiC. Metal end caps suchas zirconium or zirconium alloys or other metals can also be used. Agroup of ternary carbides or nitrides can also be used as the end capmaterials, such as Ti₂AlC, Ti₄AlN₃, Ti₃AlC₂, Ti₂SiC, Ti₃SiC₂, Ti₃SnC₂,Zr₂AlC, Zr₂TiC, Zr₂SnC, Nb₂SnC, Nb₃SiC₂, (Zr_(x)Nb_(1-x))₂AlC where0<x<1.

The most preferred ceramic composite or precursor of the same and endcaps are made from a SiC composite comprising monolithic SiC on theinside and at least one of the layers of SiC fibers on a SiC matrix.Most preferably, the end caps interface/contact points and ceramiccomposite ends are polished. The ceramic composite tube can be from 2feet to 18 feet (60.96 cm to 546.64 cm) long to accommodate a wide rangeof reactor designs.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is an enlarged longitudinal sectional view of a prior artzirconium alloy fuel rod containing fuel pellets, holding spring, andend caps;

FIG. 2 illustrates a cross sectional view of a SPS process to seal endcaps in a dual operation on both ends of a ceramic composite withoutsecondary electrodes;

FIG. 3 illustrates a cross-sectional view of a SPS process for a SPSprocess using circumferential sealing one end at a time with secondarycircumferential electrodes;

FIG. 4 illustrates a cross-sectional view of a SPS process using centralinternal and top sealing, one end cap at a time with secondarycircumferential electrodes; and

FIG. 5, which best shows the invention, illustrates a generallyschematic flow diagram of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The current and standard fuel cladding is made of various zirconiumalloys that act as the fission product barrier and prevent release ofradioactive materials to the environment. Though zirconium alloys havedesirable neutronic properties and, in the past, adequate strength andoxidation resistance in coolant at normal operation conditions, theyrapidly oxidize at beyond design basis temperatures above 1,200° C.Because the zirconium steam reaction is exothermic and rapid andhydrogen is produced during this reaction, new materials such as siliconcarbide (SiC) have been proposed and experimentally tested, which havemuch better oxidation resistance than zirconium alloys at temperaturesabove 1,200° C. Use of advanced SiC-based materials, no longer in thecomplete experimental stage, can vastly improve the fuel failuretemperature by >600° C. compared to the cladding made of zirconium alloycladding—which in itself is fantastic. This application describes asealing method that can operate a ceramic nuclear fuel tube in a nuclearreactor at elevated temperatures; and forms a radiation tolerant endseal that can be operated at >1,200° C. on fuel rod tubes made of theabove described and defined “ceramic composite.”

The “Ceramic Composite” fuel rod tube, as previously defined, is mostpreferably comprised of multiple layers of ceramic SiC materials,including dense monolithic SiC, SiC—SiC composite. Each of the layershas a specific function that contributes to meeting the performancerequirements for the cladding. In one preferred version of “ceramiccomposite” cladding, the inner layer of the cladding consists of densemonolithic SiC, which has extremely low permeability. The primaryfunction of this layer is containment of fission products. To improvereliability, the cladding may have up to three layers of monolithic SiCto provide redundant containment of fission products. Each layer will beseparated by an intermediate layer made of pyrocarbon, that is amaterial similar to graphite, but with some covalent bonding between itsgraphene sheets as a result of imperfections in its production, or othermaterial such as boron nitride or boron carbide that prevent theformation of a continuous SiC mass to inhibit crack propagation from onelayer to another. The next layer of the cladding structure is a SiC—SiCCeramic Composite.

A SiC—SiC Ceramic Composite is in tension, keeping the monolith layer incompression to counter the radial stress gradient across the clad duringperiods of high heat flux. The SiC—SiC composite can accommodate thisstress gradient because of its higher tensile stress limit compared todense, monolithic SiC. In addition, the composite layer can beengineered by adjusting the reinforcing fiber architecture. Forinstance, different braiding or winding angles can influence therelative cladding strength in the axial and hoop directions. This allowsmargin to design the most appropriate architecture to accommodate thestresses that are expected during the cladding lifetime. The outerSiC—SiC layer has the primary function of maintaining the structuralintegrity of the cladding in the event of failure of the innermonolithic SiC layer(s). All of this is again defined as being withinthe previously defined term “Ceramic Composite.” Additional SiC layersmay be added to provide additional features such as increased corrosionresistance, decreased pressure drop, increased heat transfer or otherattributes. All this is again defined as being within the previouslydefined term “Ceramic Composite.”

In this application, a vastly improved method specifically adapted tothis specific technology, involves parameter improved electric fieldassisted sintering based technology plasma sintering is used withspecific useful operating parameters disclosed discovered. Useful forthis application, to join the SiC end plug to the tubular ceramic, attheir interface; preferably a SiC-based fuel rod tube, and seal the tubewith a back filled pressure up to 500 psi; that is, spark plasmasintering (SPS), versions pertinent to this invention, are shown inFIGS. 2-4.

Alternate approaches could use plugs with opposing faces that are eitherinside the tube or outside the tube. The SPS method has a heating rateas high as 1,500° C./min. and is capable of joining two SiC pieces orpre-pieces/precursors together in a few minutes. The desirable localtemperature at the interface ranges from 1,400° C. to 2,150° C., thehold time ranges from 0.01 minutes to 60 minutes, preferably 5 minutesto 60 minutes, and the pressure ranges from 0.001 to 50 MPa, preferably,5 MPa to 20 MPa.

Turning now to the Figures, FIG. 2 illustrates use of a SPSprocess—spark plasma sintering—to apply, cement, fuse, ceramic end capsto the ceramic composite utilizing a spark plasma sintering apparatus11, here using primary and optional secondary electrodes. Also an inertgas such as He is inserted into the tubes to provide an internalbackpressure of 50 psi to 500 psi by well known processes, includingdrilling the end plugs, inserting the gas and refueling the end plugcontaining at least one ceramic composite tube 12, useful for holdingnuclear fuel pellets in a nuclear reactor, disposed between at least oneend cap 14 which end cap 14 engages the top 16 and interior 18 of theceramic composite tube, and here also the circumferential exterior 20 ofthe ceramic composite tube. A paste may be applied to the end cap 14 orsurfaces 16 and 18 to provide further sealing capability. At least twoelectrodes are attached to the end cap/tube, each electrode 22 adjacentto and in contact with the at least one end cap. The interface 26between end caps and tube ends is most preferably polished to ensurebetter adhesion.

The sintering apparatus 11 may be pressurized in vacuum or be at ambienttemperature and be in a furnace with a temperature of 50° C. up to1,500° C. Also shown are pressure control/power supply means 30, powerlines 32, fuel pellets 34 and pellet holding means, here spring 36. Theend cap(s) are preferably SiC and the preferred space of end capsenvelopment 38 is from 0.75 inch (1.905 cm) to 1.25 inch (3.175 cm).

FIG. 3, using the same numerals as FIG. 2, but using optionalcircumferential electrodes to seal one end at a time, describes a methodwhere end cap(s) have a top and circumferential contact and FIG. 4describes a method where end cap(s) have a top and interior contact. Inthese methods, there is at least one electrode 24 adjacent to and incontact with or surrounding the ceramic composite tube. FIG. 5 shows themethod of this invention where 40, a tubular ceramic composite havinginterior and exterior tube walls and a circumferential axis is supplied;the ceramic composite has attached 42 to at least one end plugcomposition, preferably the same composition as the ceramic composite:an end cap 44 covering the top, interior and sides of the ceramiccomposite—a complete but complicated sealing end cap; or an end cap 46just covering the top and sides of the ceramic composite tube shown inFIG. 3; or an end cap 48 covering the top and interior of the ceramiccomposite tube shown in FIG. 4. The end plug may be placed with orwithout paste on the end of the tube and electrodes applied to the tubeand end plug(s) and all this applied into a spark plasma sintering means(11, 50) to hermetically seal the solidified end caps securely to theend of the ceramic composite 52.

Additionally, the following features may be present:

-   -   the use of 2-18 feet (60.96 to 548.64 cm) long end-sealed fuel        rod tubes made of SiC which consists of at least one and up to        three internal layers of monolithic SiC with >95% theoretical        density, a layer made of SiC_(f)/—SiC composite and an optional        outer layer of deposited SiC;    -   the use of the fuel rod tube above sealed with monolithic SiC or        metal end plugs at one or both ends using the spark plasma        sintering method with or without SiC precursor paste material at        the joining interface, using other ceramic composite paste,        metal brazing material, glass containing materials such as        SiO₂—Al₂O₃, or metal brazing compounds such as Si and Al;    -   where the end plugs contain a circular slot with a width of        1.001 to 1.1 times of the thickness of the fuel rod tube and a        depth of 0.05 to 0.5 inches (0.127 to 1.27 cm) where the fuel        rod tube is sealed;    -   where the end plug fits inside or outside the fuel rod tube to a        depth of 0.05 to 1 inches (0.127 to 2.54 cm) providing opposing        faces where the fuel rod tube is sealed;    -   where the end plug composition/precursor material, but not the        tube, is made in one of these methods: Chemical Vapor Deposition        (CVD), cold extrusion followed by pressureless sintering, HIP,        or additive manufacturing methods such as 3D printing and laser        assisted deposition/sintering;    -   where the SiC precursor material has a density from 35% to 60%        of the theoretical value;    -   where the opposing interface surfaces of the end plug and fuel        rod tube are polished to a mirror finish or polished with a 320        grit diamond paper or finer; and    -   where the inner diameter of the fuel rod tube ranges from 0.25        to 0.60 inch (0.635 to 1.524 cm), and the thickness of the tube        ranges from 0.01 to 0.15 inch (0.025 to 0.381 cm).

This invention provides a dramatic and probably futuristic method tocommercially and practically produce a semi-flexible, controlleddestruction, disaster capable, life saving sealed 2-18 feet(60.96-548.64 cm) long semi-flexible fuel rod tubes made of monolithicSiC and SiC fiber matrix, or other “ceramic composites,” back filledwith helium or other gas up to 500 psi.

The fuel rod tube preferably has a duplex structure which consists of aninner monolithic SiC layer or multiple layers and an outer SiC/SiCpreferably composite layer. The tube is sealed with end plugs at one orboth ends made of SiC or other material. The tube and end plugs arejoined together using electric field assisted sintering technology suchas the spark plasma sintering method. The joining can be performed at anambient condition or in vacuum or in a pressurized chamber or in aheated chamber. The sealed tube is gas tight and will not deform under adifferential pressure up to 10,000 psi at up to 1,500° C. for at leastsix years.

EXAMPLE

A “Ceramic Composite” consisting of a 12 foot (365.76 cm) long extrudedstoichiometric alpha phase nuclear reactor SiC tube, for containingnuclear fuel in a reactor, with an inner diameter of 0.32 inches (0.8128cm) and a wall thickness of 0.015 inches (0.0381 cm) at a density of 95%of the SiC theoretical density, is wound with a 0.026 inch (0.066 cm)layer of SiC composite consisting of 6 layers of windings ofstoichiometric beta phase SiC fibers and is infiltrated with beta phasestoichiometric SiC using chemical vapor infiltration to a net densityfor the composite of greater than 80% of the SiC theoretical density.

This “Ceramic Composite” was sealed with extruded stoichiometric alphaphase end plugs with highly polished inner and upper tube seal faces asin FIG. 4, applied using plasma spark sintering as well known andpreviously defined in a chamber filled with helium at 375 psi at ambientcondition. The heating rate was 200° C./minute, with a bonding pressureof 5 MPa, a peak temperature of 2,100° C. at the plug/tube bondinginterface, and with a hold time of from 5 minutes up to 60 minutes atthe peak temperature.

This provided a method successfully applying end caps to the CeramicComposite SiC tubes according to the adjusted spark plasma sinteringprocess described previously; to provide a PSA sealed end tube withstandard of nuclear pressure and temperatures, and substantiallyresilient to flex and fracture.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

What is claimed is:
 1. A method of hermetically sealing at least one endcap to a nuclear reactor fuel rod cladding tube, the tube being formedfrom a ceramic composite comprising: (1) providing a tube formed from aceramic composite, the tube having tube walls, at least one open end,and a circumferential axis, and providing at least one end cap, the endcap having an exterior side and an interior side; (2) applying the atleast one end cap to the at least one open end of the tube to define aninterface between a portion of the end cap and the tube, wherein the atleast one end cap is made from a material selected from the groupconsisting of multiple layers of a SiC—SiC ceramic composite, a ternarycarbide, and a ternary nitride; (3) applying at least one primaryelectrode to the exterior side of the at least one end cap; (4) applyingcurrent to the at least one electrode using a spark plasma sinteringmeans to supply a rapid temperature rise in the interface applied for0.01 to 6.0 minutes at a rate of 200° C./min up to 1,500° C./min. wherethe temperatures at the interface are from ambient to 2,500° C.
 2. Themethod of claim 1, wherein the at least one end cap is based on SiC. 3.The method of claim 1, wherein after step (1), the portion of the endcap and tube defining the interface are polished.
 4. The method of claim1, further comprising applying in step (3) a secondary electrode to theexterior side of the tube.
 5. A method of hermetically sealing at leastone end cap to a nuclear reactor fuel rod cladding tube, the tube beingformed from a ceramic composite, comprising: (1) providing a tube formedfrom a ceramic composite, the tube having tube walls, at least one end,and a circumferential axis; (2) applying at least one end cap to the atleast one end of the tube to define an interface between a portion ofthe end cap and the tube, the end cap being formed from a compositionselected from the group consisting of a ceramic composition or aprecursor to a ceramic composition, and having an exterior and interiorside and; (3) applying at least one primary electrode to the exteriorside of the at least one end cap; (4) applying current to the at leastone electrode using a spark plasma sintering means (SPS) for 0.01 to 6.0minutes to supply a rapid temperature rise in the interface at a rate of200° C./min. up to 1,500° C./min. where temperatures at the interfaceare from ambient to 2,500° C., and holding a peak temperature for 5 to60 minutes at pressures from 0.001 MPa to 50 MPa to hermetically sealthe tube to the at least one end cap.
 6. The method of claim 5, whereinthe at least one ceramic end cap is formed from the same composition asthe composition forming the tube.
 7. The method of claim 5, wherein theoperating parameters of the SPS process are applying the rapidtemperature rise at a rate of 1000° C./min. up to 1,500° C./min. toraise the interface temperature up to 2,500° C. in 1.0 to 5.0 minutesand the pressures are 0.001 to 10 MPa.
 8. The method of claim 5, whereinthe tube and the at least one end cap are made of SiC.
 9. The method ofclaim 5, wherein after step (1), the interface of the at least one endcap and tube are polished.
 10. The method of claim 5, wherein the tubeand end cap are made from a SiC composite comprising monolithicSiC-based layer or multi-layers on the inside and at least one outerlayer of SiC-based fibers in a SiC-based matrix.
 11. The method of claim5, wherein the tube is from 2 feet to 18 feet long.
 12. The method ofclaim 5, wherein after step (1), the interface of the end caps and tubeare polished.
 13. The method of claim 5, wherein a bonding pressure of 4to 20 MPa is applied after step (3), with a hold time of 5 minutes to 60minutes applied after step (4).
 14. The method of claim 5, wherein aninert gas is inserted into the tube after step (1) or step (2), at aback pressure of 50 psi to 500 psi.
 15. The method of claim 14, whereinthe inert gas is helium.
 16. The method of claim 5, wherein at leaststep 4 of the method is carried out in a furnace having a temperature offrom about 50° C. to 1,500° C.
 17. A tubular ceramic composite made bythe method of claim
 5. 18. The method of claim 5, wherein the methodfurther comprises applying a secondary electrode to the exterior side ofthe tube and in step (4) applying current to the secondary electrode for0.01 to 6.0 minutes using the spark plasma sintering means (SPS). 19.The method of claim 1, wherein current is applied to the at least oneelectrode at a rate of about 1000° C./min to 1500° C./min. for 1.0 to5.0 minutes at a pressure of 0.001 to 10 MPa.